#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
using namespace std;


 // Definition for singly-linked list.
  struct ListNode {
      int val;
      ListNode *next;
      ListNode() : val(0), next(nullptr) {}
      ListNode(int x) : val(x), next(nullptr) {}
      ListNode(int x, ListNode *next) : val(x), next(next) {}
  };

class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
          ListNode* pre = head;
          ListNode* curr = head;
          while(curr!=nullptr){
              if(curr->val!=pre->val){
                  pre->next = curr;
                  pre = curr;
              }
              curr = curr->next;
          }
          pre->next = nullptr;
          return head;      
    }
};

